An inkjet printer is known (for example, see JP-A-2008-213162) which includes a detachable main tank, a sub tank that stores ink supplied from the mounted main tank, and an image recording unit that discharges the ink stored in the sub tank and records an image. In the inkjet printer, internal spaces of the main tank and the sub tank are opened to the air. For this reason, when the main tank is mounted on the inkjet printer, the ink moves due to a water head pressure so that the liquid level of the main tank and the liquid level of the sub tank are aligned with the same height by the difference between a water head in the internal space of the main tank and a water head in the internal space of the sub tank (hereinafter, referred to as “water head difference”). Then, the inkjet printer displays “empty” on a display or prohibits the image recording unit from discharging the ink when the residual amount of the ink detected by a residual amount detection sensor is less than a threshold.
In the inkjet printer, the discharge of the ink from the image recording unit is prohibited when the ink is stored in the sub tank so that air does not enter a flow path of the ink extending from the sub tank to the image recording unit. Thus, the inkjet printer prevents so-called air-in that air enters the flow path. On the other hand, in the inkjet printer, even when the ink stored in the main tank is completely consumed, the ink is still stored in the sub tank. Therefore, even after the ink in the main tank is consumed, it is possible to use the ink stored in the sub tank without prohibiting the discharge of the ink up to the liquid level height where air-in occurs. Since the ink can be used up the height where the air-in occurs, there is a time margin in the timing of replacing the main tank. That is, even after the ink in the main tank is consumed, image recording is enabled until the air-in occurs from the sub tank. Then, when the liquid level of the ink in the sub tank becomes a height at which the air-in may occur, the discharge of the ink from the image recording unit is prohibited.
When the main tank is replaced, the ink is discharged from the main tank to the sub tank. If the residual amount detection sensor is also provided in the sub tank, the ink flows from the main tank to the sub tank, and eventually a detection signal of the residual amount detection sensor changes. When the detection signal of the residual amount detection sensor changes, it is possible to erase the display of the empty on the display or to cancel the prohibition of the discharge of the ink. However, when the ink flows out from the main tank to the sub tank and the time is required until the signal output from the residual amount detection sensor changes, since the display of the empty on the display is not erased, a user who has replaced the main tank may presume malfunction of the device or improper replacement of the main tank. In addition, inconvenience may arise that makes the user wait until the image recording is performed after the replacement of the main tank.